The present application claims priority to Japanese Application No. P11-331494 filed Nov. 22, 1999, which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to methods of manufacturing a negative material containing silicon or a silicon compound, and of manufacturing a secondary battery provided with a negative electrode containing silicon or a silicon compound.
2. Description of the Related Art
In association with a recent miniaturization of electronic devices, a secondary battery with high energy density is required to be developed as a power source used for electric devices. As for a secondary battery in response to such requirement, for example, a lithium secondary battery using lithium (Li) metal as a negative material has been known. However, a lithium secondary battery has a defect such as a short charging-discharging cycle service life, because lithium metal is susceptible to be deposited in a negative electrode, and inactivated.
Therefore, as for a secondary battery which improves a charging-discharging cycle service life, for example, a lithium secondary battery using a negative material capable of occluding and detaching lithium ion has been developed. As for this type of negative material, such carbonaceous materials are conventionally used. Such materials are: one taken advantage of intercalation of lithium ion into interlayers, or the others adopted an occlusion and detachment effect of lithium ion in pores. Accordingly, this lithium ion secondary battery has a characteristic of a long charging-discharging cycle service life without deposition of dendrite in a negative electrode as a lithium secondary battery has. In addition, a carbonaceous material has a characteristic of stability in air and of easy-industrial production.
However, lithium ion is intercalated in the proportion of one lithium to six carbon into interlayers of a carbonaceous material. Therefore, in case of using insertion of lithium ion into interlayers, electric chemical capacity of a negative electrode has maximum limit. Additionally, in case of using occlusion or detachment of lithium ion, it is considered that control of a pore structure may increase sites, which achieves intercalation of lithium ion. However, this is a difficult technique and cannot be effective means for improvement of electric chemical capacity per volume unit and battery capacity per volume unit, because ratio of a carbonaceous material decreases.
Among carbonaceous material annealed at low temperature, there is a well-known carbonaceous material having negative electrode discharging capacity over 1000 mAh/g. This carbonaceous material has a large capacity at the rare potential over 0.8V for lithium metal. Therefore, in case that a positive electrode is formed by metal oxide, it is not a practical manner because its discharge voltage decreases. For this reason, it is difficult that currently used negative materials are applied to a longer continuous use of portable electric devices or higher energy density of power sources.
In view of the above-mentioned facts, it is suggested that a silicon compound are used as a negative material capable of occluding and detaching more lithium ion than a carbonaceous material (see JP Publication Unexamined Application HEI 10 (1998)-83817). A silicon compound has higher density compared with carbonaceous material, and can be intercalated lithium ion into its interlayers or fine voids. Because of this, using a silicon compound in a negative electrode can achieve a large charging-discharging capacity and higher energy density per unit volume. In addition, a silicon compound is also preferable material in industrial use owing to its stability in air by forming an oxide film.
Using a silicon compound in a negative electrode can achieve a large charging-discharging capacity, however, the repeat of charging and discharging seriously decreases a capacity, which causes a short charging-discharging cycle service life.
The invention has been achieved in consideration of the above problems, and its object is to provide methods of manufacturing a negative material and a secondary battery, which can achieve a large charging-discharging capacity and improvement of a charging-discharging cycle property.
A method of manufacturing a negative material according to the present invention is to manufacture a negative material containing silicon or a silicon compound, and to include a step of crushing silicon or a silicon compound in an oxygen partial pressure atmosphere within a value from higher than 10 Pa to lower than an oxygen partial pressure.
A method of manufacturing a secondary battery according to the present invention is to manufacture a secondary battery including a negative electrode containing silicon or a silicon compound, and to include a step of forming a negative electrode by a negative material containing crushed silicon or a silicon compound in an oxygen partial pressure atmosphere within a value from higher than 10 Pa to lower than an oxygen partial pressure of air.
In a method of manufacturing a negative material according to the present invention, silicon or a silicon compound is crushed in an oxygen partial pressure atmosphere within a value from higher than 10 Pa to lower than an oxygen partial pressure atmosphere of air. A secondary battery using a negative material achieved with this method improves a charging-discharging cycle property.